The present invention relates to a sputtering target, a transparent conductive film and a method for producing the same.
It is considered preferable to interpose zinc oxide type transparent conductive film between an amorphous Si layer and an Ag film as a back surface electrode of an amorphous solar cell, from the viewpoint of durability and photoelectric conversion efficiency.
However, the light absorptivity tends to be high (i.e. the light transmittance tends to decrease), when it is attempted to obtain an oxide type transparent conductive film having a low resistivity. Of a conventional zinc oxide type transparent conductive film, although the resistivity is low, the transmittance of light (particularly the transmittance of light having a wavelength of from 400 to 1000) is not high.
On the other hand, of an oxide type transparent conductive film having a low light absorptivity (i.e. a high light transmittance), the resistivity tends to be high. Accordingly, it has been difficult to obtain an oxide type transparent conductive film having a resistivity of from 10xe2x88x922 to 1010 xcexa9xc2x7cm and having a low light absorptivity.
In a case of forming an oxide type transparent conductive film having a resistivity of from 10xe2x88x922 to 1010 xcexa9xc2x7cm by a sputtering method, it has been conventionally known to carry out sputtering by using a target having such an electrical conductivity that the resistivity of the target itself is from 10xe2x88x921 to 1010 xcexa9xc2x7cm, at a low oxygen concentration such that the oxygen in the atmosphere is at most 3 vol %.
However, this method is limited to a high-frequency sputtering material, as the electrical conductivity of the target is poor. Accordingly, a direct current (DC) sputtering method can not be used which is excellent in industrial productivity such that the structure of the equipment is simple, whereby the operation efficiency is high, and the rate of film formation is high.
It is preferred to use a target of low resistance having a resistivity of at most 10xe2x88x921 xcexa9xc2x7cm when the DC sputtering method is employed, since stable discharge can be obtained.
However, in the case of forming a film by the DC sputtering method by using a ZnO type target of low resistance having a resistivity of from 10xe2x88x921 to 10xe2x88x921 xcexa9xc2x7cm, in an atmosphere of a low oxygen concentration (e.g. an argon gas alone), only a transparent conductive film of low resistance having a resistivity of a level of 10xe2x88x924 xcexa9xc2x7cm will be obtained (JP-A-2-149459).
In order to obtain a film having a resistivity of from 10xe2x88x922 to 1010 xcexa9xc2x7cm by using a target having a low resistivity of at most 10xe2x88x921 xcexa9xc2x7cm, it is required to introduce an oxidative gas (such as oxygen gas) into the atmosphere so as to carry out the sputtering at a high oxygen concentration.
However, the oxidative gas in the atmosphere may affect also the transparent conductive film to be formed, and the transparent conductive film may be damaged, such being problematic.
Accordingly, a method has been desired to produce a transparent conductive film having a resistivity of from 10xe2x88x922 to 1010 xcexa9xc2x7cm, by using the DC sputtering method which is excellent in productivity, in an atmosphere of a low oxygen concentration in which the transparent conductive film is less likely to be damaged.
It is an object of the present invention to provide a sputtering target which can be used for the DC sputtering method, and with which a transparent conductive film having a resistivity of from 10xe2x88x922 to 1010 xcexa9xc2x7cm can be stably produced.
Further, it is an object of the present invention to provide a zinc oxide type transparent conductive film having a low light absorptivity, and its production method.
Still further, it is an object of the present invention to provide a zinc oxide type transparent conductive film having a resistivity of from 10xe2x88x922 to 1010 xcexa9xc2x7cm, and its production method.
The present invention provides a sputtering target comprising an oxide containing Zn, Al and Y (hereinafter referred to as ZAY target).
By using the ZAY target, a transparent conductive film having a low light absorptivity (having substantially no coloring) will be obtained without introducing a reactive gas such as oxygen during sputtering (e.g. by an argon gas alone).
The resistivity of the ZAY target is preferably at most 10xe2x88x921 xcexa9xc2x7cm. If it is higher than 10xe2x88x921 xcexa9xc2x7cm, the discharge during the DC sputtering tends to be unstable.
By using the ZAY target, an oxide type transparent conductive film having a resistivity of from 10xe2x88x922 to 1010 xcexa9xc2x7cm will be obtained stably.
The ZAY target contains preferably from 0.2 to 15.0 atomic % of Al and from 0.2 to 75.0 atomic % of Y, based on the total amount of Zn, Al and Y, since a transparent conductive film having a resistivity of from 10xe2x88x922 to 1010 xcexa9xc2x7cm will be obtained more stably. Particularly, in order to obtain a transparent conductive film having a resistivity of from 10xe2x88x922 to 104 xcexa9xc2x7cm, the ZAY target contains preferably from 0.2 to 8.0 atomic % of Al and from 0.2 to 20.0 atomic % of Y, more preferably from 0.2 to 8.0 atomic % of Al and from 0.2 to 10.0 atomic % of Y, based on the total amount of Zn, Al and Y.
In a case of an oxide target containing from 0.2 to 15.0 atomic % of Al based on the total amount of Zn, Al and Y, and containing no Y, when the DC sputtering is carried out in an atmosphere at a low oxygen concentration (e.g. an argon gas alone), of the obtained transparent conductive film, the resistivity will be less than 10xe2x88x922 xcexa9xc2x7cm, however, the light absorptivity tends to be high.
Further, in a case of an oxide target containing from 0.2 to 75.0 atomic % of Y based on the total amount of Zn, Al and Y, and containing no Al, the target will have a high resistance (exceeding 10xe2x88x921 xcexa9xc2x7cm), whereby the DC sputtering tends to be difficult. Further, in a case of using, instead of Al, B of the same Group 3 of Periodic Table, the target tends to be poor in moisture resistance.
By properly adjusting the amount of Al and Y, a transparent conductive film having a desired resistivity of from 10xe2x88x922 to 1010 xcexa9xc2x7cm will be obtained.
Further, the atomic ratio of Y/Al in the ZAY target is preferably from 0.1 to 25. If the atomic ratio of Y/Al is less than 0.1, the light absorptivity of the obtained transparent conductive film tends to be high. On the other hand, if the atomic ratio of Y/Al exceeds 25, the resistivity of the obtained transparent conductive film tends to exceed 1010 xcexa9xc2x7cm.
The method for producing the ZAY target is not particularly limited, and a common method for sintering ceramics such as atmospheric pressure sintering method or a hot pressing method may be employed.
The atmospheric pressure sintering method is preferred since a sintered body (target) can be produced at a low cost without using a huge equipment such as a vacuum equipment.
The ZAY target can be obtained, for example, by mixing each powder of alumina, yttria and zinc oxide as materials to obtain a powder mixture, filling the powder mixture in a mold for pressing, followed by press molding by a press machine or a rubber press machine (or molding by plaster casting), and then sintering the obtained molded product at from 1400 to 1550xc2x0 C. in the air.
If it is less than 1400xc2x0 C., the sintering degree will be poor, whereby no dense target will be obtained. Further, if it is higher than 1550xc2x0 C., the evaporation from the sintered body will be active, and pores in the sintered body will be enlarged by the vaporized gas, whereby the density tends to decrease. The holding time during sintering is preferably from 1 to 10 hours. If it is shorter than 1 hour, the sintering tends to be inadequate, whereby no dense product tends to be obtained. Further, if it is longer than 10 hours, the amount of evaporation tends to be large, whereby the density tends to decrease.
The density of the ZAY target is preferably at least 85% as a relative density, since stable discharge can be obtained.
Another element other than Al and Y may be added to the ZAY target. As said another element, lanthanoid such as La or Ce, or Sc, may, for example, be mentioned.
The ZAY target preferably has a Y3Al5O12 phase as a garnet structure. By formation of the Y3Al5O12 phase, solid solution of Y in large quantities in ZnO can be suppressed, and as a result, decrease in electrical conductivity can be suppressed.
Further, the ZnAl2O4 phase as a spinel structure is preferably as little as possible. The electrical conductivity tends to be high when the formation of the ZnAl2O4 phase is little, and when Al is solid-solved in ZnO. Concretely, the peak intensity of the (311) face in the ZnAl2O4 phase is smaller than the peak intensity of the (101) face of the Y3Al5O12 phase, in a powder X-ray diffraction analysis.
The present invention further provides a method for producing a transparent conductive film, which comprises sputtering a sputtering target, wherein the ZAY target is used as the sputtering target.
The sputtering atmosphere is preferably such an atmosphere that the content of an oxidative gas is at most 3 vol %. An atmosphere of an inert gas such as an argon gas alone may also be employed.
If the content of an oxidative gas in the atmosphere exceeds 3 vol %, the oxidative gas in the atmosphere may re-sputter the formed transparent conductive film, whereby the transparent conductive film may be damaged.
As the oxidative gas in the present invention, a gas having oxygen atoms such as O2, H2O, CO or CO2, may be mentioned.
As a sputtering method, any discharge system such as a DC system or a high frequency system may be carried out, however, preferred is a DC sputtering method having a simple equipment structure and thereby having a good operation efficiency, having a high rate of film formation and being excellent in industrial productivity.
As a substrate on which the film is formed, glass, ceramics, a plastic or a metal may, for example, be mentioned. The temperature of the substrate during film formation is not particularly limited. Further, after the film formation, the substrate may be subjected to after-heating (heat treatment).
The present invention further provides a transparent conductive film comprising an oxide containing Zn, Al and Y (hereinafter referred to as ZAY film).
The ZAY film contains preferably from 0.2 to 15.0 atomic % of Al and from 0.2 to 75.0 atomic % of Y, based on the total amount of Zn, Al and Y, since a desired resistivity will be obtained with a high reproducibility. Particularly, it is preferred to contain from 0.2 to 8.0 atomic % of Al and from 0.2 to 60.0 atomic % of Y.
In order to obtain a resistivity of from 10xe2x88x922 to 104 xcexa9xc2x7cm, it is particularly preferred to contain from 0.2 to 8.0 atomic % of Al and from 0.2 to 20.0 atomic % of Y, based on the total amount of Zn, Al and Y.
Further, from the viewpoint of light absorptivity, the sum of Al and Y is preferably at least 3.5 atomic %, particularly preferably at least 5 atomic %, based on the total amount of Zn, Al and Y.
In a case where from 0.2 to 15.0 atomic % of Al is contained based on the total amount of Zn, Al and Y, and no Y is contained, the resistivity tends to be less than 10xe2x88x922 xcexa9xc2x7cm, and the light absorptivity tends to be high.
Further, in a case where from 0.2 to 75.0 atomic % of Y is contained based on the total amount of Zn, Al and Y, and no Al is contained, the resistivity tends to exceed 1010xc2x7cm. Further, in a case where B of the same Group 3 of Periodic Table is used instead of Al, the film tends to be poor in moisture resistance.
By properly adjusting the amount of Al and Y, a ZAY film having a desired resistivity of from 10xe2x88x922 to 1010 xcexa9xc2x7cm will be obtained.
The atomic ratio of Y/Al in the ZAY film is preferably from 0.1 to 25. If the atomic ratio of Y/Al is less than 0.1, the light absorptivity tends to be high. On the other hand, if the atomic ratio of Y/Al exceeds 25, the resistivity tends to exceed 1010 xcexa9xc2x7cm. From the viewpoint of resistivity, the atomic ratio of Y/Al is particularly preferably at most 5.
From the viewpoint of light absorptivity, the atomic ratio of Y/Al is preferably at least 0.5, particularly preferably at least 1.0.
Another element other than Al and Y may be added to the ZAY film. As said another element, lanthanoid such as La or Ce, or Sc, may, for example, be mentioned.
The film thickness (geometrical film thickness) of the ZAY film is preferably from 5 nm to 1.5 xcexcm. If it is thinner than 5 nm, no continuous film tends to be obtained. Further, if it is thicker than 1.5 xcexcm, the cost tends to increase, and at the same time, peeling is likely to occur. From the viewpoint of light absorption, the film thickness is particularly preferably from 5 to 200 nm.
With the ZAY film, a relatively high resistivity of from 10xe2x88x922 to 1010 xcexa9xc2x7cm can be easily achieved, and it is suitable for an application, for example, which requires a high sheet resistance value of from 103 to 1011 xcexa9/xe2x96xa1. It is particularly suitable for a transparent heating element to obtain a desired heat value. The electrical conductivity in the vicinity of the electrode area is practically very important, and the electrical conductivity in the vicinity of the electrode is sufficient with a resistance of from 103 to 1011 xcexa9/xe2x96xa1, and the current can be stably applied to the film (the film can stably be used), and extraordinary heat generation in the vicinity of the electrode can be prevented.
Further, if the resistivity of the ZAY film is lower than 10xe2x88x922 xcexa9xc2x7cm, the film thickness is required to be thin in order to obtain a desired sheet resistance value (e.g. from 103 to 1011 xcexa9/xe2x96xa1), and accordingly, problems such as breakage of the film are likely to arise.
On the other hand, if the resistivity of the ZAY film is higher than 1010 xcexa9xc2x7cm, the film thickness is required to be thick in order to obtain a desired sheet resistance value (from 103 to 1011 xcexa9/xe2x96xa1), and accordingly, the cost tends to increase.
The present invention further provides a transparent conductive film-bonded substrate which comprises a substrate and a ZAY film formed on the substrate. Specific examples of the transparent conductive film-bonded substrate having a ZAY film formed thereon, include the above-described transparent heating element, a solar cell (as described hereinafter) and a touch panel.
When the ZAY film is used as a base film for another transparent conductive film, and another transparent conductive film is laminated on said ZAY film, the crystallinity of the transparent conductive film laminated on the ZAY film can be adjusted. Namely, in the ZAY film, the C axis of the zinc oxide crystal is aligned perpendicular to the substrate, which may affect the crystallinity of the transparent conductive film laminated on the ZAY film. The transparent conductive film laminated on the ZAY film will be affected by the crystal alignment of the ZAY film as the base film, and will be a transparent conductive film having a high crystallinity due to a phenomenon similar to epitaxial growth. This is particularly remarkable in a case where the transparent conductive film laminated on the ZAY film is a zinc oxide type transparent conductive film.
Further, as a result of increase in crystallinity, influence of moisture remaining in the vacuum (contamination) will be suppressed during film formation, and besides, characteristics such as electrical conductivity and durability tend to be better.
Further, in the case where the ZAY film is used as the base film, when the substrate is soda lime glass, diffusion of alkali ions from the substrate can be prevented, and when the substrate is a plastic film, evaporation or diffusion of a solvent or a low molecular weight substance contained in the film can be prevented.
Further, in a case where the ZAY film is used as an overcoat film for another transparent conductive film, it works as a protective layer to prevent invasion of moisture or oxygen from the air. Further, by lamination with another transparent conductive film, the internal stress of the laminated film can be reduced, and the breakage of the film can be reduced.
As mentioned above, when the ZAY film is used as the base film and/or overcoat film, the electrical conductivity of another transparent conductive film will be maintained for a long period of time. For example, in a case where the ZAY film and another transparent conductive film are combined (laminated) to constitute a transparent conductive film-bonded substrate, the change in the sheet resistance value of said transparent conductive film-bonded substrate will be small even after a long period of time.
In the ZAY film, as the carrier electron concentration in the film is reduced by addition of Y, the absorption of light at the visible light and near infrared regions due to carrier electrons is small, and as a result, the light transmittance can be increased. Further, by addition of Y, the defect level (interstitial zinc) in the ZAY film can be reduced, whereby an action such that the bandgap becomes narrow due to the defect level can be suppressed, and as a result, the light absorption of visible light on the near ultraviolet ray side (from 380 to 500 nm) can be reduced, whereby the light transmittance can be increased.
The absorptivity ((absorptivity)=100xe2x88x92(transmittance)xe2x88x92(reflectance)) of the ZAY film at each wavelength of from 400 to 1000 nm is preferably at most 4% (extruding the absorption of the substrate). If it is higher than 4%, the light absorption is significant, such being unfavorable practically. Particularly, when it is used for a solar cell, the photoelectric conversion efficiency tends to decrease, and when it is used for a transparent article such as a transparent heating element, the transparency tends to be impaired.
The present invention further provides a solar cell which comprises a substrate, and a front surface electrode, a photoelectric conversion layer and a back surface electrode formed on the substrate in this order, wherein the ZAY film is formed between the photoelectric conversion layer and the back surface electrode.
In a thin-film solar cell such as an amorphous Si solar cell, for example, when the ZAY film is used as a transparent conductive layer to be interposed between an amorphous Si layer and a back surface electrode made of a metal (e.g. Ag film), decrease in power generation efficiency can be minimized, since the light absorption of the ZAY film at the visible light region and the near infrared region is small, and the light energy will be used more efficiently.
Al in the ZAY film and the ZAY target will be solid-solved in ZnO in part or in whole, and work to form electrons which develop the electrical conductivity.
On the other hand, Y will work to decrease the carrier concentration, and to decrease the mobility as an impurity scattering source. Further, Y in the ZAY target will work also as a sintering aid.
By the actions of Al and Y, a transparent conductive film having a resistivity of from 10xe2x88x922 to 1010 xcexa9xc2x7cm, and having a low light absorptivity, can be obtained even by sputtering at a low oxygen concentration.
Y in the ZAY film is uniformly dispersed in the film, and is present in such a state that it is solid-solved in ZnO. On the other hand, Y in the ZAY target is present as segregated, and the above-mentioned action by Y to increase the resistance will cover only a narrow area, and accordingly, the entire ZAY target will have a low resistance of a level of 1031 xcexa9xc2x7cm. Y in the ZAY target is present as Y2O3 or in such a state that it is solid-solved in ZnO.